core: shared notes on the desktop and phone, and Share from the desktop

The core pulls with shares from a server offering them (protocol 6): a note
says how it is held (owner, edit, view) and who shared it, and a revoked note
leaves the device. The first such pull starts the feed over once, so notes
shared before this build arrive. The store refuses what a share doesn't allow
(view: everything; edit: anything but the text), push sends only the text of
someone else's note, and their notes stay out of trash, reminders and
reordering. Unlinking drops them.

The Share dialog's calls go to the linked server over the device token, as
Tauri commands and through the FFI. The desktop now offers Share and "Shared
with me"; unlinked, the dialog says sharing needs a server.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

core/src/local/models.rs

@@ -28,6 +28,20 @@ pub struct Note {
     pub previews: Vec<LinkPreview>,
     pub created_at: Option<String>,
     pub updated_at: Option<String>,
+    /// How this account holds the note (#5175): `owner`, or `edit`/`view` for one
+    /// someone shared with it. Named and valued as the server's, so the shared
+    /// frontend gates the editor identically either way.
+    pub permission: String,
+    /// Whether the owner has shared it with anyone.
+    pub shared: bool,
+    /// Who shared it with us; null on our own notes.
+    pub shared_by: Option<SharedBy>,
+}
+
+#[derive(Serialize)]
+pub struct SharedBy {
+    pub id: String,
+    pub display_name: String,
 }
 
 #[derive(Serialize)]
@@ -173,4 +187,7 @@ pub struct Facets {
     pub created_after: Option<String>,
     #[serde(default)]
     pub created_before: Option<String>,
+    /// `with_me`: only notes someone else shared with this account.
+    #[serde(default)]
+    pub shared: Option<String>,
 }